Barite–pyrite mineralization of the Wiesbaden thermal spring system, Germany: a 500-kyr record of geochemical evolution

Barite–(pyrite) mineralizations from the thermal springs of Wiesbaden, Rhenish Massif, Germany, have been studied to place constraints on the geochemical evolution of the hydrothermal system in space and time. The thermal springs, characterized by high total dissolved solids (TDS) contents and predominance of NaCl, ascend from aquifers at 3–4 km depth and discharge at a temperature of 65–70°C. The barite–(pyrite) mineralization is found in upflow and discharge zones of the present‐day thermal springs as well as at elevations up to 50 m above the current water table. Hence, this mineralization style constitutes a continuous record of the hydrothermal activity, linking the past evolution with the present state of this geothermal system. The sulphur isotope signatures of the mineralization indicate a continuous decrease of the δ³⁴S of sulphate from +16.9‰ in the oldest barite to +10.1‰ in the present‐day thermal water. The δ³⁴S values of barite closely resemble various recently active thermal springs along the southern margin of the Rhenish Massif and contrast strongly with different regional ground and mineral waters. The mineralogical and isotopic signatures, combined with calculations based on uplift rates and the regional geological history, indicate a minimum activity of the thermal spring system at Wiesbaden of about 500 000 years. This timeframe is considerably larger than conservative models, which estimate the duration of thermal spring systems in continental intraplate settings to last for several 10 000 years. The calculated equilibrium sulphur isotope temperatures of coexisting barite and pyrite range between 65 and 80°C, close to the discharge temperature of the springs, which would indicate apparent equilibrium precipitation. Kinetic modelling of the re‐equilibration of the sulphate–sulphide pair during water ascent shows that this process would require 220 Myr. Therefore, we conclude that pyrite is formed from precursor Fe monosulphide phases, which rapidly precipitate in the near‐surface environment, preserving the isotope fractionation between dissolved sulphate and sulphide established in the deep aquifer. Equilibrium modelling of water–mineral reactions shows slight supersaturation of barite at the discharge temperature. Pyrite is already strongly supersaturated at the temperatures estimated for the aquifer (110°C) and processes in the near‐surface environment are most probably related to contact of the thermal water with atmospheric oxygen, resulting in formation of oxidized intermediate sulphur species and precipitation of Fe monosulphide phases, which subsequently recrystallize to pyrite.     

Geochemistry of the Bagnères-de-Bigorre thermal waters from the North Pyrenean Zone (France)

Thermal springs are poorly known in the sedimentary sites of the Pyrenees. In this paper, we describe the ‘Bagnères‐de‐Bigorre’ springs which occur in a remarkably active seismotectonic context. A chemical and isotopic study of 15 spring waters (both cold and thermal, ranging in temperature from 7.0 to 49.9°C), and continuous monitoring of a single spring allow us to characterise water–rock interactions, fluids paths and mixing processes. Three groups of waters are distinguished: (I) SO₄²⁺–Ca²⁺–Cl^– thermal waters (II) SO₄²⁺–Cl^––Ca²⁺ thermal waters and (III) HCO₃^––Ca²⁺ cold shallow waters. Their characteristics suggest interactions with Mesozoic evaporite and carbonate formations. O and D isotopes from thermal waters indicate a local meteoric origin of Atlantic signature and a recharge elevation of 800 to 1000 m, which corresponds to a single feeding area. Their δ¹³C values (?2.8 to ?9.6‰) are consistent with carbonate dissolution, slight fractionation and a surficial organic input leading to δ¹³C depletion. Sr isotopes (0.70751 to 0.70777), Na⁺/Cl^– and (Ca²⁺ + Mg²⁺)/SO₄^2– ratios as well as thermodynamic calculations show that the dissolution of anhydrite and halite‐bearing Triassic layers control the chemical composition of group‐I and ‐II waters. The contrasting trends of cation/Cl^– ratios and TDS of waters from groups I and II suggest the existence of two different circulation paths at depth as well as dilution with surficial waters similar to group III. Calculated mixing proportions show that three waters from group I are diluted from 17 to 66%, whereas all waters from group II are mixed. The aquifer temperature is estimated to be in the range 55–64°C using the retrograde and prograde solubilities of anhydrite and chalcedony, respectively. Accordingly, the mean depth of the reservoir is around 1.7 km, which allows us to constrain the depth of the Triassic layer.     

Hydrogeochemical dynamics affecting steam‐heated pools at El Chichón Crater (Chiapas – Mexico)

El Chichón is an active volcano located in the north‐western Chiapas, southern Mexico. The crater hosts a lake, a spring, named Soap Pool, emerging from the underlying volcanic aquifer and several mud pools/hot springs on the internal flanks of the crater which strongly interact with the current fumarolic system (steam‐heated pools). Some of these pools, the crater lake and a cold spring emerging from the 1982 pumice deposits, have been sampled and analysed. Water–volcanic gas interactions determine the heating (43–99°C) and acidification (pH 2–4) of the springs, mainly by H₂S oxidation. Significantly, in the study area, a significant NH₃ partial pressure has been also detected. Such a geochemically aggressive environment enhances alteration of the rock in situ and strongly increases the mineralization of the waters (and therefore their electrical conductivity). Two different mineralization systems were detected for the crater waters: the soap pool‐lake (Na⁺/Cl^? = 0.4, Na/Mg>10) and the crater mud pools (Na⁺/Cl^? > 10, Na/Mg < 4). A deep boiling, Na⁺‐K⁺‐Cl^?‐rich water reservoir generally influences the Soap Pool‐lake, while the mud pool is mainly dominated by water‐gas–rock interactions. In the latter case, conductivity of sampled water is directly proportional to the presence of reactive gases in solution. Therefore, chemical evolution proceeds through neutralization due to both rock alteration and bacterial oxidation of ammonium to nitrate. The chemical compositions show that El Chichón aqueous fluids, within the crater, interact with gases fed by a geothermal reservoir, without clear additions of deep magmatic fluids. This new geochemical dataset, together with previously published data, can be used as a base line with which to follow‐up the activity of this deadly volcano.     

Infiltration of basinal fluids into high-grade basement, South Norway: sources and behaviour of waters and brines

Quartz veins hosted by the high‐grade crystalline rocks of the Modum complex, Southern Norway, formed when basinal fluids from an overlying Palaeozoic foreland basin infiltrated the basement at temperatures of c. 220°C (higher in the southernmost part of the area). This infiltration resulted in the formation of veins containing both two‐phase and halite‐bearing aqueous fluid inclusions, sometimes with bitumen and hydrocarbon inclusions. Microthermometric results demonstrate a very wide range of salinities of aqueous fluids preserved in these veins, ranging from c. 0 to 40 wt% NaCl equivalent. The range in homogenization temperatures is also very large (99–322°C for the entire dataset) and shows little or no correlation with salinity. A combination of aqueous fluid microthermometry, halogen geochemistry and oxygen isotope studies suggest that fluids from a range of separate aquifers were responsible for the quartz growth, but all have chemistries comparable to sedimentary formation waters. The bulk of the quartz grew from relatively low δ¹⁸O fluids derived directly from the basin or equilibrated in the upper part of the basement (T < 200°C). Nevertheless, some fluids acquired higher salinities due to deep wall‐rock hydration reactions leading to salt saturation at high temperatures (>300°C). The range in fluid inclusion homogenization temperatures and densities, combined with estimates of the ambient temperature of the basement rocks suggests that at different times veins acted as conduits for influx of both hotter and colder fluids, as well as experiencing fluctuations in fluid pressure. This is interpreted to reflect episodic flow linked to seismicity, with hotter dry basement rocks acting as a sink for cooler fluids from the overlying basin, while detailed flow paths reflected local effects of opening and closing of individual fractures as well as reaction with wall rocks. Thermal considerations suggest that the duration of some flow events was very short, possibly in the order of days. As a result of the complex pattern of fracturing and flow in the Modum basement, it was possible for shallow fluids to penetrate basement rocks at significantly higher temperatures, and this demonstrates the potential for hydrolytic weakening of continental crust by sedimentary fluids.     

The salt chimney effect: delay of thermal evolution of deep hydrocarbon source rocks due to high thermal conductivity of evaporites

Half of the topseals to the world's largest oilfields are evaporites. Rock salt has a thermal conductivity two to four times greater than that of other sedimentary rocks found in oil‐ and gas‐bearing basins. Strong heat conduction through evaporites can increase the geothermal gradient above evaporite deposits, resulting in a positive thermal anomaly and above‐average temperature while simultaneously decreasing the geothermal gradient below evaporites, resulting in a negative thermal anomaly. Most Triassic–Jurassic hydrocarbon source rocks in the Kuqa Basin, western China, are overlain by ~1500‐m‐thick Tertiary evaporites with underlying Cretaceous sandstones and mudstones. Directly measured strata temperatures indicate an obvious break in the steepness of the geothermal gradient above and below Paleogene evaporites, with a significantly steeper geothermal gradient above the evaporites. Simulations of the thermal evolution of source rocks based on data collected from well Kela‐2 indicate that if the thickness of evaporites (mainly rock salt and anhydrite rock) in overlying rocks above source rocks increases compared with the thickness of siliciclastic rocks in the overlying rocks, then strata temperatures and vitrinite reflectance in Jurassic source rocks will decrease accordingly. Our thermal simulations based on the thickness and thermal conductivity of evaporites accurately coincide with previous studies based on homogenization temperatures, hydrocarbon–water contact retrospection, and carbon isotope results from natural gases. The gas generation center located in the Kalasu Tectonic Belt today is also sealed in an evaporite‐related structural trap that formed at this time. Therefore, the speculated natural gas generation times not only correlate with the evaporite‐related structural trap formation, but the calculated maturity of deep source rocks below the evaporites also coincides with current gas reserves. And our studies can help to find the deep oils and gases under thick evaporites.     

Deep geothermal groundwater flow in the Seferihisar–Balçova area,Turkey: results from transient numerical simulations of coupled fluid flow and heat transport processes

The Seferihisar–Balçova Geothermal system (SBG) is characterized by complex temperature and hydrochemical anomalies. Previous geophysical and hydrochemical investigations suggest that hydrothermal convection in the faulted areas of the SBG and recharge flow from the Horst may be responsible for the observed patterns. A numerical model of coupled fluid flow and heat transport processes has been built in order to study the possible fluid dynamics of deep geothermal groundwater flow in the SBG. The results support the hypothesis derived from interpreted data. The simulated scenarios provide a better understanding of the geophysical conditions under which the different fluid dynamics develop. When recharge processes are weak, the convective patterns in the faults can expand to surrounding reservoir units or below the seafloor. These fault‐induced drag forces can cause natural seawater intrusion. In the Melange of the Seferihisar Horst, the regional flow is modified by buoyant‐driven flow focused in the series of vertical faults. As a result, the main groundwater divide can shift. Sealing caprocks prevent fault‐induced cells from being overwhelmed by vigorous regional flow. In this case, over‐pressured, blind geothermal reservoirs form below the caprocks. Transient results showed that the front of rising hot waters in faults is unstable: the tip of the hydrothermal plumes can split and lead to periodical temperature oscillations. This phenomenon known as Taylor–Saffman fingering has been described in mid‐ocean ridge hydrothermal systems. Our findings suggest that this type of thermal pulsing can also develop in active, faulted geothermal systems. To some extent, the role of an impervious fault core on the flow patterns has been investigated. Although it is not possible to reproduce basin‐scale transport processes, this first attempt to model deep groundwater geothermal flow in the SBG qualitatively supported the interpreted data and described the different fluid dynamics of the basin. Geofluids (2010) 10 , 388–405     

Geothermal fluids circulation at Caldas do Moledo area, Northern Portugal: geochemical and isotopic signatures

A hydrogeological conceptual model of the Caldas do Moledo geothermal site is proposed that shows mixing between geothermal waters and local shallow groundwaters. Stable isotope values of Caldas do Moledo geothermal waters indicate recharge areas located at relatively high altitudes (850–1250 m a.s.l.). The NW–SE Vigo–Régua shear zone plays an important role in fluid recharge and circulation towards the NNE–SSW Régua–Verin fault system, forming a path for ascent of geothermal fluids. The apparent ¹⁴C age of geothermal fluids (15.66 ± 2.86 ka BP) was estimated in the total dissolved inorganic carbon (TDIC). Geothermometer calculations indicate that, assuming a conductive temperature gradient of 32°C per kilometer for northern Portugal, the maximum depth of circulation is roughly 1.8 ± 0.4 km. The K, Ca and SO₄ concentrations found in some Caldas do Moledo geothermal spring waters show mixing between deep geothermal and shallow groundwater systems. Local shallow groundwaters showing the highest SO₄ concentrations were found at low elevation areas, originating from fertilisers and pesticides applied to the Port wine vineyards in the Douro River valley. Geothermal waters from boreholes AC1 and AC2 do not show evidences of direct pollution from the spreading of such agrochemicals.     

Thermal springs and heat flow in North America

Thermal springs are commonly thought to be an indicator of geothermal resource potential. However, there have been few analyses of the relationship between thermal springs and the underlying thermal regime. An examination of temperature and discharge rates for a large database of thermal springs in North America demonstrates that there is not a simple relationship between these measurements made at the surface and subsurface heat flow. Hydrogeological factors appear to exert strong controls on the temperature and discharge at these springs and should be carefully considered in geothermal resource assessments.     

Inferring intermediate‐scale fluid flow in a heterogeneous metasedimentary multilayer sequence during progressive deformation: evidence from the Monts d’Arrée slate belt (Brittany,France)

Quartz veins in the early Variscan Monts d’Arrée slate belt (Central Armorican Terrane, Western France), have been used to determine fluid‐flow characteristics. A combination of a detailed structural analysis, fluid inclusion microthermometry and stable isotope analyses provides insights in the scale of fluid flow and the water–rock interactions. This research suggests that fluids were expelled during progressive deformation and underwent an evolution in fluid chemistry because of changing redox conditions. Seven quartz‐vein generations were identified in the metasedimentary multilayer sequence of the Upper Silurian to Lower Devonian Plougastel Formation, and placed within the time frame of the deformation history. Fluid inclusion data of primary inclusions in syn‐ to post‐tectonic vein generations indicate a gradual increase in methane content of the aqueous–gaseous H₂O–CO₂–NaCl–CH₄–N₂ fluid during similar P–T conditions (350–400°C and 2–3.5 kbar). The heterogeneous centimetre‐ to metre‐scale multilayer sequence of quartzites and phyllites has a range of oxygen‐isotope values (8.0–14.1‰ Vienna Standard Mean Ocean Water), which is comparable with the range in the crosscutting quartz veins (10.5–14.7‰ V‐SMOW). Significant differences between oxygen‐isotope values of veins and adjacent host rock (Δ = ?2.8‰ to +4.9‰ V‐SMOW) suggest an absence of host‐rock buffering on a centimetre scale, but based on the similar range of isotope values in the Plougastel Formation, an intraformational buffering and an intermediate‐scale fluid‐flow system could be inferred. The abundance of veins, their well‐distributed and isolated occurrence, and their direct relationship with the progressive deformation suggests that the intermediate‐scale fluid‐flow system primarily occurred in a dynamically generated network of temporarily open fractures.     

Origin of palaeo‐waters in the Ordovician carbonates in Tahe oilfield,Tarim Basin: constraints from fluid inclusions and Sr,C and O isotopes

Petrographic features, isotopes, and trace elements were determined, and fluid inclusions were analyzed on fracture‐filling, karst‐filling and interparticle calcite cement from the Ordovician carbonates in Tahe oilfield, Tarim basin, NW China. The aim was to assess the origin and evolution of palaeo‐waters in the carbonates. The initial water was seawater diluted by meteoric water, as indicated by bright cathodoluminescence (CL) in low‐temperature calcite. The palaeoseawater was further buried to temperatures from 57 to 110°C, nonluminescent calcite precipitated during the Silurian to middle Devonian. Infiltration of meteoric water of late Devonian age into the carbonate rocks was recorded in the first generation of fracture‐ and karst‐filling dull red CL calcite with temperatures from <50°C to 83°C, low salinities (<9.0 wt%), high Mn contents and high ⁸⁶Sr/⁸⁷Sr ratios from 0.7090 to 0.7099. During the early Permian, ⁸⁷Sr‐rich hydrothermal water may have entered the carbonate rocks, from which precipitated a second generation of fracture‐filling and interparticle calcite and barite cements with salinities greater than 22.4 wt%, and temperatures from 120°C to 180°C. The hydrothermal water may have collected isotopically light CO₂ (possibly of TSR‐origin) during upward migration, resulting in hydrothermal calcite and the present‐day oilfield water having δ¹³C values from ?4.3 to ?13.8‰ and showing negative relationships of ⁸⁷Sr/⁸⁶Sr ratios to δ¹³C and δ¹⁸O values. However, higher temperatures (up to 187°C) and much lower salinities (down to 0.5 wt%) measured from some karst‐filling, giant, nonluminescent calcite crystals may suggest that hydrothermal water was deeply recycled, reduced (Fe‐bearing) meteoric water heated in deeper strata, or water generated from TSR during hydrothermal water activity. Mixing of hydrothermal and local basinal water (or diagenetically altered connate water) with meteoric waters of late Permian age and/or later may have resulted in large variations in salinity of the present oilfield waters with the lowest salinity formation waters in the palaeohighs.     

Fracture mineralization and fluid flow evolution: an example from Ordovician–Devonian carbonates,southwestern Ontario,Canada

Petrography, geochemistry (stable and radiogenic isotopes), and fluid inclusion microthermometry of matrix dolomite, fracture‐filling calcite, and saddle dolomite in Ordovician to Devonian carbonates from southwestern Ontario, Canada, provide useful insights into fluid flow evolution during diagenesis. The calculated δ¹⁸O_fluid, ΣREE, and REE_SN patterns of matrix and saddle dolomite suggest diverse fluids were involved in dolomitization and/or recrystallization of dolomite. The ⁸⁷Sr/⁸⁶Sr ratios of dolomite of each succession vary from values in the range of coeval seawater to values more radiogenic than corresponding seawater, which indicate diagenetic fluids were influenced by significant water/rock interaction. High salinities (22.4–26.3 wt. % NaCl + CaCl₂) of Silurian and Ordovician dolomite–hosted fluid inclusions indicate involvement of saline waters from dissolution of Silurian evaporites. High fluid inclusion homogenization temperatures (>100°C) in all samples from Devonian to Ordovician show temperatures higher than maximum burial (60–90°C) of their host strata and suggest involvement of hydrothermal fluids in precipitation and/or recrystallization of dolomite. A thermal anomaly over the mid‐continent rift during Devonian to Mississippian time likely was the source of excess heat in the basin. Thermal buoyancy resulting from this anomaly was the driving force for migration of hydrothermal fluids through regional aquifers from the center of the Michigan Basin toward its margin. The decreasing trend of homogenization temperatures from the basin center toward its margin further supports the interpreted migration of hydrothermal fluids from the basin center toward its margin. Hydrocarbon‐bearing fluid inclusions in late‐stage Devonian to Ordovician calcite cements with high homogenization temperatures (>80°C) and their ¹³C‐depleted values (approaching ?32‰ PDB) indicate the close relationship between hydrothermal fluids and hydrocarbon migration.     

Retrograde P–T evolution and high temperature–low pressure fluid circulation in relation to late Hercynian intrusions: a mineralogical and fluid inclusion study of the Charroux-Civray plutonic complex (north-western Massif Central, France)

The calc‐alkaline plutonic complex from Charroux‐Civray (north‐western part of the French Massif Central) displays multiphase hydrothermal alteration. Plutonic rocks, as well as early retrograde Ca–Al silicate assemblages, which have crystallized during cooling and uplifting of the plutonic series, are affected by multiphase chlorite–phengite–illite–carbonate alteration linked to intense pervasive fluid circulation through microfractures. The petrographic study of alteration sequences and their associated fluid inclusions in microfissures of the plutonic rocks, as well as in mineral fillings of the veins, yields a reconstruction of the P–T–X evolution of the Hercynian basement after the crystallization of the main calc‐alkaline plutonic bodies. This reconstruction covers the uplift of the basement to its exposure and the subsequent burial by Mesozoic sediments. Cooling of the calc‐alkaline plutonic series started at solidus temperatures (～650°C), at a pressure of about 4 kbar (1 bar = 10⁵ N m^?2), as indicated by magmatic epidote stability, hornblende barometry and fluid inclusion data. Cooling continued under slightly decreasing pressure during uplift down to 2–3 kbar at 200–280°C (prehnite–pumpellyite paragenesis). Then, a hot geothermal circulation of CO₂‐bearing fluids was induced within the calc‐alkaline rocks leading to the formation of greisen‐like mineralizations. During this stage, temperatures around 400–450°C were still high for the inferred depths (～2 kbar). They imply abnormal heat flows and thermal gradients of 60–80°C km^?1. The hypothesis of the existence of one large or a succession of smaller peraluminous plutons at depth, supported by geophysical data, suggests that localized heat flows were linked to concealed leucogranite intrusions. As uplift continued, greisen mineralization was subsequently affected by the chlorite–phengite–dolomite assemblage, correlated with aqueous and nitrogen‐bearing fluid circulations in the temperature range of 400–450°C. In a later stage, a continuous temperature decrease at constant pressure (～0.5 kbar) led to the alteration of the dolomite–illite–chlorite type in the 130–250°C temperature range.     

Hydrogeology of the Krafla geothermal system,northeast Iceland

The Krafla geothermal system is located in Iceland's northeastern neovolcanic zone, within the Krafla central volcanic complex. Geothermal fluids are superheated steam closest to the magma heat source, two‐phase at higher depths, and sub‐boiling at the shallowest depths. Hydrogen isotope ratios of geothermal fluids range from ?87‰, equivalent to local meteoric water, to ?94‰. These fluids are enriched in ¹⁸O relative to the global meteoric line by +0.5–3.2‰. Calculated vapor fractions of the fluids are 0.0–0.5 wt% (~0–16% by volume) in the northwestern portion of the geothermal system and increase towards the southeast, up to 5.4 wt% (~57% by volume). Hydrothermal epidote sampled from 900 to 2500 m depth has δD values from ?127 to ?108‰, and δ¹⁸O from ?13.0 to ?9.6‰. Fluids in equilibrium with epidote have isotope compositions similar to those calculated for the vapor phase of two‐phase aquifer fluids. We interpret the large range in δD_EPIDOTE and δ¹⁸O_EPIDOTE across the system and within individual wells (up to 7‰ and 3.3‰, respectively) to result from variable mixing of shallow sub‐boiling groundwater with condensates of vapor rising from a deeper two‐phase reservoir. The data suggest that meteoric waters derived from a single source in the northwest are separated into the shallow sub‐boiling reservoir, and deeper two‐phase reservoir. Interaction between these reservoirs occurs by channelized vertical flow of vapor along fractures, and input of magmatic volatiles further alters fluid chemistry in some wells. Isotopic compositions of hydrothermal epidote reflect local equilibrium with fluids formed by mixtures of shallow water, deep vapor condensates, and magmatic volatiles, whose ionic strength is subsequently derived from dissolution of basalt host rock. This study illustrates the benefits of combining phase segregation effects in two‐phase systems during analysis of wellhead fluid data with stable isotope values of hydrous alteration minerals when evaluating the complex hydrogeology of volcano‐hosted geothermal systems.     

Pyrophyllite formation in the thermal aureole of a hydrothermal system in the Lower Saxony Basin,Germany

A combined clay mineralogical, fluid inclusion, and K‐Ar study of Upper Jurassic metasediments at the Gehn (Lower Saxony Basin, Germany) provides evidence for a transient hydrothermal event during Upper Cretaceous basin inversion centered on a prominent gravimetric anomaly. Kaolinite and smectite in Oxfordian pelitic parent rocks that cap a deltaic sandstone unit were locally transformed into pyrophyllite, 2M₁ illite, R3 illite–smectite, chlorite, and berthierine at the Ueffeln quarry. The pyrophyllite‐bearing metapelites lack bedding‐parallel preferred orientation of sheet silicates and experienced peak temperatures of about 260–270°C consistent with microthermometric data on quartz veins in the underlying silicified sandstones. The presence of expandable layers in illite–smectite and high Kübler Index values indicate that the thermal event was rather short‐lived. K‐Ar dating of the <0.2 μm fraction of the pyrophyllite‐bearing Ueffeln metapelite yields a maximum illitization age of 117 ± 2 Ma. Lower trapping temperatures of aqueous fluid inclusions in quartz veins and the absence of pyrophyllite in metapelites of the Frettberg quarry in a distance of about 2.5 km from the Ueffeln quarry infer maximum paleotemperatures of only 220°C. The highly localized thermal anomaly at Ueffeln suggests fault‐controlled fluid migration and heat transfer that provided a thermal aureole for pyrophyllite formation in the metapelites rather than metamorphism due to deep burial. A pH neutral hydrothermal fluid that formed by devolatilization reactions or less likely by mixing of meteoric and marine waters that interacted at depth with shales is indicated by the low salinity (3–5 wt. % NaCl equiv.) of aqueous inclusions, their coexistence with methane–carbon dioxide‐dominated gas inclusions as well as carbon, hydrogen, and oxygen isotope data. The upwelling zone of hydrothermal fluids and the thermal maximum is centered on a gravimetric anomaly interpreted as an igneous intrusion (‘Bramsche Massif’) providing the heat source for the intrabasinal hydrothermal system.     

Evaluation of soluble benzene migration in the Uinta Basin

Field sampling and mathematical modeling are used to study the long‐distance transport and attenuation of petroleum‐derived benzene in the Uinta Basin, Utah. Benzene concentration was measured from oil and oil field formation waters of the Altamont‐Bluebell and Pariette Bench oil fields in the basin. It was also measured from springs located in the regional groundwater discharge areas, hydraulically down‐gradient from the oil fields sampled. The average benzene concentration in oils and co‐produced waters is 1946 and 4.9 ppm at the Altamont‐Bluebell field and 1533 and 0.6 ppm at the Pariette Bench field, respectively. Benzene concentration is below the detection limit in all springs sampled. Mathematical models are constructed along a north–south trending transect across the basin through both fields. The models represent groundwater flow, heat transfer and advective/dispersive benzene transport in the basin, as well as benzene diffusion within the oil reservoirs. The coupled groundwater flow and heat transfer model is calibrated using available thermal and hydrologic data. We were able to reproduce the observed excess fluid pressure within the lower Green River Formation and the observed convective temperature anomalies across the northern basin. Using the computed best‐fit flow and temperature, the coupled transport model simulates water washing of benzene from the oil reservoirs. Without the effect of benzene attenuation, dissolved benzene reaches the regional groundwater discharge areas in measurable concentration (>0.01 ppm); with attenuation, benzene concentration diminishes to below the detection limit within 1–4 km from the reservoirs. Attenuation also controls the amount of water washing over time. In general, models that represent benzene attenuation in the basin produce results more consistent with field observations.     

Chemical and isotopic signatures of Na/HCO3/CO2‐rich geofluids,North Portugal

Geochemical and isotopic studies have been undertaken to assess the origin of CO₂‐rich waters issuing in the northern part of Portugal. These solutions are hot (76°C) to cold (17°C) Na–HCO₃ mineral waters. The δ²H and δ¹⁸O signatures of the mineral waters reflect the influence of altitude on meteoric recharge. The lack of an ¹⁸O‐shift indicates there has been no high temperature water–rock interaction at depth, corroborating the results of several chemical geothermometers (reservoir temperature of about 120°C). The low ¹⁴C activity (up to 9.9 pmC) measured in some of the cold CO₂‐rich mineral waters (total dissolved inorganic carbon) is incompatible with the presence of ³H (from 1.7 to 4.1 TU) in those waters, which indicates relatively short subsurface circulation times. The δ¹³C values of CO₂ gas and dissolved inorganic carbon range between ?6‰ and ?1‰ versus Vienna‐Peedee Belemnite, indicating that the total carbon in the recharge waters is being diluted by larger quantities of CO₂ (¹⁴C‐free) introduced from deep‐seated (upper mantle) sources, masking the ¹⁴C‐dating values. The differences in the ⁸⁷Sr/⁸⁶Sr ratios of the studied thermal and mineral waters seem to be caused by water–rock interaction with different granitic rocks. Chlorine isotope signatures (?0.4‰ < δ³⁷Cl < +0.4‰ versus standard mean ocean chloride) indicate that Cl in these waters could be derived from mixing of a small amount of igneous Cl from leaching of granitic rocks.     

Response of Alum Rock springs to the October 30, 2007 Alum Rock earthquake and implications for the origin of increased discharge after earthquakes

The origins of increased stream flow and spring discharge following earthquakes have been the subject of controversy, in large part because there are many models to explain observations and few measurements suitable for distinguishing between hypotheses. On October 30, 2007 a magnitude 5.5 earthquake occurred near the Alum Rock springs, California, USA. Within a day we documented a several‐fold increase in discharge. Over the following year, we have monitored a gradual return towards pre‐earthquake properties, but for the largest springs there appears to be a permanent increase in discharge. The Alum Rock springs discharge waters that are a mixture between modern (shallow) meteoric water and old (deep) connate waters expelled by regional transpression. After the earthquake, there was a small and temporary decrease in the fraction of connate water in the largest springs. Accompanying this geochemical change was a small (1–2°C) temperature decrease. Combined with the rapid response, this implies that the increased discharge has a shallow origin. Increased discharge at these springs occurs both for earthquakes that cause static volumetric expansion and for those that cause contraction, supporting models in which dynamic strains are responsible for the subsurface changes that cause flow to increase. We make a quantitative comparison between the observed changes and model predictions for three types of models: (i) a permanent increase in permeability; (ii) an increase in permeability followed by a gradual decrease to its pre‐earthquake value; and (iii) an increase of hydraulic head in the groundwater system discharging at the springs. We show that models in which the permeability of the fracture system feeding the springs increases after the earthquake are in general consistent with the changes in discharge. The postseismic decrease in discharge could either reflect the groundwater system adjusting to the new, higher permeability or a gradual return of permeability to pre‐earthquake values; the available data do not allow us to distinguish between these two scenarios. However, the response of these springs to another earthquake will provide critical constraints on the changes that occur in the subsurface and should permit a test of all three types of models.     

Origin of dolomites in the Cambrian (upper 3rd‐Furongian) formation,south‐eastern Sichuan Basin,China

The origin of large‐scale ancient dolomite is one of the most hotly debated topics in sedimentology. The Loushanguan group of the upper 3rd‐Furongian Cambrian series on the south‐eastern margin of the Sichuan Basin consists of numerous dolomites, and the origins of these dolomites have never been reported previously although they are probably good hydrocarbon reservoirs. Based on a systematic analysis of petrology, fluid inclusions, carbon and oxygen isotopes, trace elements and rare earth elements (REEs), this study provides some unique insights into the origins of the dolomites. Four dolomite types have been identified in the study area: dolomicrite, fabric‐retentive oolitic dolomite, fabric‐obliterative dolomite and saddle dolomite cement. In the dolomicrite and fabric‐retentive oolitic dolomite, high Sr contents (with respect to the fabric‐obliterative dolomite) and the lack of two‐phase aqueous inclusions suggest that they formed at shallow‐to‐intermediate burial depths at low temperatures (<50–60°C). Carbon and oxygen isotopes and seawater‐like REE+Y characteristics of the dolomicrite and fabric‐retentive oolitic dolomite indicate that the dolomitizing fluids were evaporated seawater or slightly modified seawater. The obliteration of the original sedimentary fabric and relatively low δ¹⁸O and Sr values compared to the fabric‐retentive dolomite indicate that fabric‐obliterative dolomite formed at intermediate‐to‐deep burial diagenesis. The chemical composition approaches pure dolomite and the REE+Y characteristics are similar to those of the fabric‐retentive dolomite, indicating that the fabric‐obliterative dolomite was formed due to the recrystallization of the previously formed fabric‐retentive dolomite at elevated burial depths and temperatures. High fluid inclusion homogenization temperatures (115–150°C), low δ¹⁸O values, nonplanar‐a crystals and seawater‐like REE+Y characteristics suggest that saddle dolomite cement formed by reprecipitation of dolomite that related to seawater‐driven and deep burial fluid. In the study area, dolomicrite and fabric‐retentive oolitic dolomite may have been formed by penecontemporaneous or seepage‐reflux dolomitization during early‐stage diagenesis. Subsequently, during progressive burial, most of the fabric‐retentive dolomite was converted into fabric‐obliterative dolomite by recrystallization. This study confirms that fabric‐obliterative dolomite was the main dolomite type, and although deeply buried, these Cambrian dolomites most likely have preserved coeval seawater geochemical signals.     

Hydrothermal monitoring in a quiescent volcanic arc: Cascade Range,northwestern United States

Ongoing (1996–present) volcanic unrest near South Sister, Oregon, is accompanied by a striking set of hydrothermal anomalies, including elevated temperatures, elevated major ion concentrations, and ³He/⁴He ratios as large as 8.6 R_A in slightly thermal springs. These observations prompted the US Geological Survey to begin a systematic hydrothermal‐monitoring effort encompassing 25 sites and 10 of the highest‐risk volcanoes in the Cascade volcanic arc, from Mount Baker near the Canadian border to Lassen Peak in northern California. A concerted effort was made to develop hourly, multiyear records of temperature and/or hydrothermal solute flux, suitable for retrospective comparison with other continuous geophysical monitoring data. Targets included summit fumarole groups and springs/streams that show clear evidence of magmatic influence in the form of high ³He/⁴He ratios and/or anomalous fluxes of magmatic CO₂ or heat. As of 2009–2012, summit fumarole temperatures in the Cascade Range were generally near or below the local pure water boiling point; the maximum observed superheat was <2.5°C at Mount Baker. Variability in ground temperature records from the summit fumarole sites is temperature‐dependent, with the hottest sites tending to show less variability. Seasonal variability in the hydrothermal solute flux from magmatically influenced springs varied from essentially undetectable to a factor of 5–10. This range of observed behavior owes mainly to the local climate regime, with strongly snowmelt‐influenced springs and streams exhibiting more variability. As of the end of the 2012 field season, there had been 87 occurrences of local seismic energy densities approximately ≥ 0.001 J/m³ during periods of hourly record. Hydrothermal responses to these small seismic stimuli were generally undetectable or ambiguous. Evaluation of multiyear to multidecadal trends indicates that whereas the hydrothermal system at Mount St. Helens is still fast‐evolving in response to the 1980–present eruptive cycle, there is no clear evidence of ongoing long‐term trends in hydrothermal activity at other Cascade Range volcanoes that have been active or restless during the past century (Baker, South Sister, and Lassen). Experience gained during the Cascade Range hydrothermal‐monitoring experiment informs ongoing efforts to capture entire unrest cycles at more active but generally less accessible volcanoes such as those in the Aleutian arc.     

Regional groundwater flow and interactions with deep fluids in western Apennine: the case of Narni‐Amelia chain (Central Italy)

The elemental fluxes and heat flow associated with large aquifer systems can be significant both at local and at regional scales. In fact, large amounts of heat transported by regional groundwater flow can affect the subsurface thermal regime, and the amount of matter discharged towards the surface by large spring systems can be significant relative to the elemental fluxes of surface waters. The Narni‐Amelia regional aquifer system (Central Italy) discharges more than 13 m³ sec^?1 of groundwater characterised by a slight thermal anomaly, high salinity and high pCO₂. During circulation in the regional aquifer, groundwater reacts with the host rocks (dolostones, limestones and evaporites) and mixes with deep CO₂‐rich fluids of mantle origin. These processes transfer large amounts of dissolved substances, in particular carbon dioxide, and a considerable amount of heat towards the surface. Because practically all the water circulating in the Narni‐Amelia system is discharged by few large springs (Stifone‐Montoro), the mass and energy balance of these springs can give a good estimation of the mass and heat transported from the entire system towards the surface. By means of a detailed mass and balance of the aquifer and considering the soil CO₂ fluxes measured from the main gas emission of the region, we computed a total CO₂ discharge of about 7.8 × 10⁹ mol a^?1 for the whole Narni‐Amelia system. Finally, considering the enthalpy difference between infiltrating water and water discharged by the springs, we computed an advective heat transfer related to groundwater flow of 410 ± 50 MW.